Multi-axis lens, beam system making use of the compound lens, and method of manufacturing the compound lens

ABSTRACT

The invention provides a lens system for a plurality of charged particle beams. Therein, at least one common excitation coil for at least two lens modules is provided. The lens modules comprise an first pole piece, a second pole piece and at least one opening for a charged particle beam. The lens modules constitute a component and share the excitation coil. Thereby, raw material availability, processing of work pieces and symmetry conditions for the lens fields are improved.

FIELD OF THE INVENTION

The invention relates to an optical system for multiple beam chargedparticle applications, such as inspection system applications, testingsystem applications, lithography system applications and the like. Italso relates to a lens system for a plurality of charged particle beamsand to methods of manufacturing thereof. In particular, the inventionrelates to a lens system for a plurality of charged particle beamscomprising modules, more particularly to a lens system for multi-beamapplications. Specifically, it relates to a lens system for a pluralityof charged particle beams, a method of manufacturing a lens system and acharged particle beam device.

BACKGROUND OF THE INVENTION

Charged particle beam apparatuses are used in a plurality of industrialfields. Testing of semiconductor devices during manufacturing, exposuresystems for lithography, detecting devices and inspection systems areonly some of these fields.

In general, there is a high demand for structuring and inspectingspecimens within the micrometer or nanometer scale. On such a smallscale, process control, inspection or structuring is often done withcharged particle beams, e.g. electron beams, which are generated andfocused in charged particle beam devices, such as electron microscopes,electron beam pattern generators or charged particle inspection systems.Charged particle beams offer superior spatial resolution compared to,e.g. photon beams due to their short wavelengths.

However, for a given beam diameter, the charged particle beam currentlimits the throughput of charged particle beam systems. Since furtherminiaturization of, e.g. structures to be imaged is necessary, thecharged particle beam diameter has to be decreased. As a result, thebeam current for individual beams and thus, the throughput has to bedecreased.

In order to increase the total charged particle beam current and thus,the throughput, a plurality of charged particle beams can be used. Oneoption for a system applying a plurality of charged particle beams is tocombine several single beam columns with each other. However, some ofthe components, especially magnetic lenses, cannot be miniaturizedsufficiently, since the magnetic field cannot be arbitrarily increased.Thus, the columns have to be spaced such that the electron beams have adistance of 100 mm to 200 mm.

To overcome this problem, U.S. Pat. No. 3,715,580 utilizes a magneticlens with a circular excitation coil providing two holes, each for oneelectron beam. Thereby, the continuous rotation symmetry of previouslenses is abandoned since the hole (optical axis) for each electron beamhas different distances from the position of the excitation.

Patent application US 2001/0028038A1 shows an excitation coil common toa plurality of holes in a pole piece. To increase the number of electronbeams that can be used, US 2001/0028038A1 uses a two-dimensional array.To compensate for the differences with respect to the focusingproperties of individual beams, this prior art teaches to use lensintensity adjusters.

Since there is a strong requirement for improving resolution, forsimplifying manufacturing and for minimizing aberrations in suchsystems, it is an object of the present invention to further improvestate of the art devices.

SUMMARY OF THE INVENTION

The present invention intends to provide an improved lens system for aplurality of charged particles. Thereby, one object is to improve thesymmetry of the lens field used for imaging the charged particle beam.Another object is to provide an advantageous manufacturing method forthe lens systems. Further, a lens system according to independent claims1 and 21, a manufacturing method according to independent claim 17 and amultiple charged particle beam device according to independent claim 34are provided.

Further advantages, features, aspects and details of the invention areevident from the dependent claims, the description and the accompanyingdrawings.

According to one aspect of the present invention, a lens system for acharged particle column is provided. The lens system comprises at leasttwo lens modules. The at least two lens modules share a single commonexcitation coil. Each lens module comprises a first and second polepiece and an opening for a charged particle beam.

Making use of the at least two lens modules with a common excitationcoil, the size of the work pieces of the magnetic-conductive materialcan be reduced. Thus, the required size of raw material, which isdifficult to obtain in large pieces, is reduced. Further, demands onmanufacturing tolerances can more easily be realized with smaller workpieces attained from the raw material. Additionally, the lens propertiesof the individual lenses are more uniform with respect to each other.

According to a preferred aspect, a magnetic lens field, provided foreach optical axis, has an n-fold symmetry with 1<n<∞. Preferably, a2-fold symmetry with respect to the optical axis is realized.

Alternatively, the symmetry can be described as having two planes ofsymmetry, or as having no dipole moment.

Thus, a magnetic lens system for a plurality of charged particle beamsis provided. The lens system has an n-fold symmetry with 1<n<∞ withrespect to each optical axis. Prior art systems only provide a similarsymmetry with respect to the entire lens for a plurality of beams. Thus,the lens system according to an aspect of the invention does not have aweak continuous rotational symmetry, but a strong n-fold symmetryinstead. In this context, weak or strong symmetry should be understoodas follows. Generally, lens systems intend to provide a lens field witha defined symmetry. This symmetry is more or less distorted bymanufacturing tolerances or by limitations to the lens design. A weaksymmetry is to be understood as a rough approximation of the desiredsymmetry. Thus, the weak symmetry has a lot of distortion, since, e.g.openings for a charged particle beam are not concentric with a circularexcitation coil. A strong symmetry is to be understood as closeapproximation of the desired symmetry. Thus, a strong symmetry has onlylittle distortion due to manufacturing imperfections or the like. Thewell-defined non-continuous rotational symmetry according to an aspectof the invention can more easily be corrected, since the symmetry of thefield influencing the charged particle beam is known.

According to a further preferred aspect, the lens modules are arrangedsuch that the openings form a linear lens array. That is, the openingsform a lens row. It is especially preferred when at least four openingsin one row are provided. In this context, a lens row is referred to asseveral openings for charged particle beams positioned to formsubstantially a line. By increasing the number of openings within onerow, a quasi-infinitely long row of openings is formed. In this context,a quasi-infinitely long linear lens array is a row of lens openings witha length such that most of the openings are influenced by magneticfields of the surrounding as if the linear lens array is infinitelylong. Thus, the necessity to avoid cross talk between individual lensmodules is reduced, since for a quasi-infinitely long linear lens array,each opening has a similar influencing periphery.

Providing the lens modules such that a linear lens array is formed, itis further preferred when the excitation coil is non-circular, and morepreferably if it is rectangular with rounded edges. As described above,the lens module is shaped to provide a magnetic field with an n-foldsymmetry. Furthermore, the excitation coil, as well as the opening forthe charged particle beam influences the symmetry of the magnetic lensfiled. If, according to the described preferred aspect, the excitationcoil is formed non-circular, the intended symmetry can be similarlyapplied to the excitation coil. Therefore the desired symmetry for theentire system can be obtained more easily. Generally, an excitation coilhas several windings. Having a rectangular coil with rounded edges astadium-like form is realized, especially if the edges are rounded, suchthat at two ends a semi-circle is realized.

According to another preferred aspect, the lens modules are positionedsuch that adjacent optical axes have a distance between 10 mm to 90 mm,preferably between 30 mm to 65 mm. Thus, a distance between chargedparticle beams can be achieved which is smaller than a distance betweencharged particle beams in the case when two individual charged particlebeam columns are located next to each other. Consequently, inspection,lithography or testing applications for a plurality of charged particlebeams with an increased density of charged particle beams can berealized.

According to a further preferred aspect, adjacent lens modules areprovided with a gap of about 0.1 mm to 3 mm. More preferably, the gap isfilled with a non-magnetic material. Thus, a separation of theindividual lens modules can be realized and thus cross talk can beavoided or significantly reduced. As a result, other lens modules do notdistort the symmetry of the individual lens modules.

According to another aspect of the present invention, the lens module ismanufactured according to a method comprising the following steps:manufacturing a plurality of lens modules, and providing a commonexcitation coil for at least two lens modules. Thus, the size ofmagnetic-material work pieces that have to be processed is reduced.Making use of this aspect, it is preferred when a cylindricalintermediate work piece is formed. Thus, processing is furthersimplified and manufacturing tolerances can be further decreased.

According to another aspect of the present invention, a lens system fora plurality of charged particle beams is provided. The lens systemcomprises a pole piece unit with first pole piece, a second pole pieceand at least four openings for charged particle beams and an excitationcoil. Thereby, the openings are arranged in one row, forming a lens row.According to a preferred aspect, the lens system comprises at leastseven openings in one row. Thus, the symmetry of the magnetic lens fieldwith respect to each individual opening is approximately equal to thesymmetry of an infinitely long lens row. Consequently, at least twoplanes of symmetry can be obtained.

According to a preferred aspect, the excitation coil has a rectangularshape with rounded edges. Thus, the symmetry of the excitation coilcorresponds to the desired symmetry of the magnetic lens field.

According to a preferred aspect, a lens system is provided comprising atleast two lens rows each comprising an excitation coil. The lens rowsare arranged next to each other to form a two-dimensional arrangement ofopenings. Thus, the number of charged particle beams and the throughputof the measuring system can be increased.

The invention is also directed to apparatus for carrying out thedisclosed methods including apparatus parts for performing eachdescribed method steps. These method steps may be performed by way ofhardware components, a computer programmed by appropriate software, byany combination of the two or in any other manner. Furthermore, theinvention is also directed to methods by which the described apparatusoperates or is manufactured. It includes method steps for carrying outevery function of the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

Some of the above indicated and other more detailed aspects of theinvention will be described in the following description and partiallyillustrated with reference to the figures. Therein:

FIGS. 1 a to 1 c show schematic views of the build-up of an embodimentof an lens system according to the invention;

FIG. 2 shows a schematic view of a further embodiment of a lens systemaccording to the invention;

FIGS. 3 to 5 show schematic views of further embodiments of lensarrangements according to the invention;

FIGS. 6 a to 6 b show schematic views of further embodiments accordingto the invention including possibilities to avoid cross talk;

FIGS. 7 a to 7 b show schematic views of further embodiments accordingto the invention, including means for providing conditions for aquasi-infinitely long lens row;

FIG. 8 shows a schematic view of a further embodiment according to theinvention, including dummy openings for flux shaping;

FIG. 9 shows a schematic side view of an embodiment according to theinvention, including an electrostatic lens component;

FIGS. 10 a to 11 b show schematic top views of embodiments according tothe invention having different types of symmetry;

FIG. 12 shows a schematic top view of an embodiment according to theinvention, explaining the influence of the excitation on the symmetry;

FIGS. 13 to 15 show schematic top views of further embodiments of linearlens arrays and the combination of the linear lens arrays for atwo-dimensional lens array;

FIGS. 16 to 17 show schematic views of further embodiments according toa further aspect of the invention;

FIGS. 18 to 19 show schematic views of a circular magnetic lens for asingle charged particle beam; and

FIGS. 20 and 21 show schematic views of possibilities to build a lenssystem for a plurality of charged particle beams.

DETAILED DESCRIPTION OF THE DRAWINGS

Without limiting the scope of protection of the present application, inthe following the charged particle multi-beam device will exemplarily bereferred to as an electron multi-beam device. Thereby, electron beamdevice with a plurality of electron beams might especially be anelectron beam inspection system. The present invention can still beapplied for apparatuses using other sources of charged particles forinspection, testing and lithography applications, and in the case ofdetection devices other secondary charged particles to obtain a specimenimage or the like.

In the following, a state of the art optic for focusing a plurality ofelectron beams will be described with reference to FIGS. 18-19. Amagnetic lens 20 is rotationally symmetrical with respect to opticalaxis 24. This rotational symmetry is indicated with arrow 29 in FIG. 18.The magnetic lens consists of an excitation coil 21 and amagnetic-conductive circuit 23 which confines the magnetic field to thegap region between upper pole piece 25 and lower pole piece 27.According to different shapes of lens systems, other pole piecesarrangements can be realized. Radial gap lenses, for example, have aninner and an outer pole piece. Therefore, generally speaking, a lenssystem comprises a first and a second pole piece, forming a gap regionto confine the magnetic field. The field in the gap region interactswith an electron beam traveling through opening 22. In the case of aperfect rotationally symmetrical lens and an electron beam traveling onthe optical axis 24 (=axis of symmetry), the electron beam will beinfluenced by the field without introducing astigmatism.

FIG. 19 shows a 3-dimensional view of a rotationally symmetricalmagnetic lens. The magnetic-conductive circuit 23 forms the body of thelens. An electron beam travels through opening 22. Thereby, the magneticfield focuses the electron beam. Due to the symmetry, aberrations areminimized if the beam travels on the optical axis 24.

FIG. 20 shows several circular lenses located next to each other. Thedistance between two optical axes 24 is denoted as L. The onlypossibility to further decrease the distance between two neighboringelectron beams is to decrease the size of the lens. This however, wouldresult in a reduction of the field strength and thus the imagingproperties of the lenses.

FIG. 21 shows a further attempt to provide a lens for two electronbeams. One circular magnetic circuit 23 is provided with two openingsforming a gap region wherein the magnetic field interacts with theelectron beam. A problem of such a system can be understood whenreferring to the different distances d1 and d2. The distances d1 and d2indicate two arbitrary distances which are measured along a directionradial to optical axis 24. As can be seen, the edge of themagnetic-conductive circuit body has different distances to the opticalaxis. The magnetic-conductive circuit 23 of the lens consists ofpermalloy or μ-metal or any other magnetic conductive material. Thesmaller the magnetic resistance of the material is, the less influenceswill be introduced by the distance of the excitation of the magneticcircuit and the field-beam-interaction region. Because of the non-zeromagnetic resistance of the magnetic-conductive material of the polepiece unit 23, the magnetic field around openings 22 shows a gradient.Thus, the magnetic fields influencing each electron beam is notrotational symmetric with respect to each individual optical axis.

To minimize this effect, the magnetic lens circuit can be enlarged,thereby obtaining smaller relative differences with respect to themagnetic resistances. Thus, the amount of distortion of the symmetry canbe reduced by enlarging the pole piece area between the location ofexcitation an the location in which the magnetic field acts on theelectron beam. However, such an approach is in conflict withmanufacturing aspects. For example, the availability of magneticmaterials can be limited for large pieces. Further, as shown in FIG. 21the lens system is not continuously rotationally symmetrical, ascompared to an embodiment of FIG. 19. Therefore, the lens system can notbe manufactured with a lathe or the like. Thus, the low tolerancemanufacturing of an embodiment according to FIG. 21 is more delicate.

The present invention intends to provide a more sophisticatedmanufacturing method and apparatus. Thereby, several options can berealized. As can be seen, there is a desire to realize multi-electronlenses. Further, the distance between the optical axes of the electronbeams should be decreased. However, the attempt according to FIG. 21reduces the continuous rotational symmetry with respect to eachindividual optical axis without increasing a different type of symmetry,such as n-fold rotational symmetries or symmetries with respect to aplurality of planes. In general, due to the asymmetry introduced,astigmatism is increased.

The present invention provides a lens that gives up a weak continuousrotational symmetry in favor of a strong n-fold symmetry, whereby n isat least two. Thus, there is no attempt to obtain a continuousrotational symmetry. Instead, a lens system is provided which has amagnetic field for each electron beam that is theoretically free of adipole moment. In this context, weak or strong symmetry should beunderstood as follows. Generally, lens systems intend to provide a lensfield with a defined symmetry. This symmetry is more or less distortedby manufacturing tolerances or by limitations to the lens design. A weaksymmetry is to be understood as a rough approximation of the desiredsymmetry. Thus, the weak symmetry has a lot of distortion, since, e.g.openings for a charged particle beam are not concentric with a circularexcitation coil. A strong symmetry is to be understood as closeapproximation of the desired symmetry. Thus, a strong symmetry has onlylittle distortion due to manufacturing imperfections or the like.

Thereby, it is possible to work with lens modules, that is to say a lensconsisting of several magnetic-conductive pieces. Thus, the size of therequired magnetic raw material is reduced. Further, manufacturingprocesses can be improved.

One embodiment according to the present invention will now be describedwith respect to FIGS. 1 a to 1 c. FIG. 1 a shows three lens modules 100.The lens module has cylinder-like form comprising two convex sides 111and two flat sides 110. The two opposite sides 110 form two parallelflats. Sides 110 are not entirely closed, but have a part not covered byan external wall. Therefore, excitation coil 21 can protrude out of themagnetic-conductive circuit 23 in FIG. 1 a. The lens module has twoplanes of symmetry 102 and 104, intersecting on optical axis 24. Opening22 is concentric to optical axis 24. Thus, excited by the coil, eachlens module 100 has a magnetic lens field, which is symmetrical to plane102 and 104. The symmetry can also be described as a 2-fold symmetrywith respect to the optical axis 24. What could further be described asforming a lens module such that the magnetic lens field has no dipolemoment and a defined quadrupole moment.

To bring the electron beams of an electron device closer together, thelens modules 100 can be located side by side. Additionally, the circularcoils 21 are replaced by a common excitation device 106 with straightconductors. The symmetry of each optical axis or opening 22 is notaffected by this modification of the excitation introduced in FIG. 1 bcompared to FIG. 1 a.

FIG. 1 c shows five lens modules 100 located directly side by side. Theelectrical flux inducing the magnetic fields is indicated as 106 a and106 b. Having the electrical current continued to each side, every lensmodule 100 of FIG. 1 c influences an electron beam on its optical axis24 according to the same excitation (cross talk and screeningdisregarded) and with the same dipole-less symmetry, already describedabove.

The distance of the five optical axes 24 in FIG. 1 c could besignificantly reduced. This distance is in the range of 40 mm instead ofabout 100 mm (see “L” in FIG. 20)

According to a preferred manufacturing process, the embodiment shown inFIGS. 1 a to 1 c is manufactured from circular magnetic-conductivecircuits having circular openings which are flattened at two oppositesides to obtain two parallel flats. The modules (lens modules 100) arelocated side by side, putting the flattened sides close together. Thus,most of the manufacturing steps can be conducted with small rotationallysymmetrical work pieces. Thus, the precision required for good imagingproperties can be achieved easier, since a lathe or the like can be usedfor manufacturing most of the modules.

Another embodiment according to the invention is shown in FIG. 2. Lensmodules 200 are not manufactured from a circular work piece but from arectangular work piece. Also opening 202 penetrating themagnetic-conductive circuit 203 has an rectangular shape. Opening 202comprises a hole in the upper and in the lower pole piece of themagnetic-conductive circuit. Two conductors 206 in the form of straightwires are used to excite the lens modules and thereby generate a lensfield. The lens field has, similar to FIGS. 1 a to 1 c, a 2-foldsymmetry with respect to the optical axis. Further, each module and thelens field, respectively, have two planes of symmetry, which intersectthe optical axis 24. In other words, there is no dipole moment.

In FIG. 2, the lens modules 200 are spaced by distance G. G can forexample be between 0.1 to 3 mm. Within FIG. 2, distance G is forgraphical reasons drawn to be larger. Distance G has the followingeffect. Conductive material circuit 203 has a low magnetic resistance.Air (or any non-magnetic material) in the gap between the lens modules200 has a high magnetic resistance. Thus, each lens module can beconsidered as having no neighboring lens module. Due to the highmagnetic resistance within the gap (distance G), cross talk can beavoided or significantly decreased.

Further embodiments of the present invention will next be described withrespect to FIGS. 3 to 5. Therein, the magnetic-conductive circuits 301,shown in FIG. 2, are used as lens modules without any space in-between.Thus, there might be cross talk to the neighboring lens module 301.However, a lens module, e.g. module 301 c, is influenced by adjacentmodules 301 b and 301 d. Influences by modules 301 a and 301 e and,especially, even further distant modules can be neglected. Thus, as longas the linear lens array 300 is long enough, most of the lens modules301 are influenced by the same number of neighboring modules. For thisreason, the symmetry arguments provided above, remain valid.

However, lens modules 301 positioned at the end of lens array 300 do nothave a symmetrical periphery. Thus, lens modules positioned at the endof the lens array should either not be used for focusing an electronbeam or the effect of not having a neighboring lens module should becompensated for. This will be explained below with respect to FIG. 7.

FIG. 3 additionally differs from FIG. 2 by providing differently shapedopenings 302 for the electron beam. Rectangular openings 202 in FIG. 2are replaced by oval openings 302. As described above, the intention ofthe present invention is not to have a continuous rotation symmetry thatis strongly distorted. Instead, an n-fold symmetry, which iswell-defined and is theoretically free of a dipole moment, is aimed for.In this context, theoretically free is to be understood according to thefollowing. The magnetic field is free of a dipole moment besidessymmetry-distortions induced by manufacturing intolerances or anapproximation at an end of a linear lens array. In general, however, forlens modules having two neighbors, one at each side the dipole momentcan be neglected.

The symmetry of the magnetic field interacting with the electron beamdepends on the symmetry of the magnetic-conductive circuit, of theexcitation and also on the symmetry of opening 302. In view of thisinfluence, opening 302 should be designed to give a well-definednon-continuous rotational symmetry. Within FIGS. 2 to 5, rectangularopenings 202, which can optionally have rounded edges, oval openings 302and 402 oriented along different planes of symmetry or circular openings502, can for example be provided. The modification of the shape of theopenings described above can be combined with other features accordingto different embodiments presented.

The embodiments shown in FIGS. 4 and 5 additionally differ from FIG. 3in the number of openings provided per lens module. FIG. 4 shows a lensmodule 403 for one electron beam, a lens module 404 for two electronbeams and a lens module 405 for three electron beams. In general, thedifferent lens modules can be combined. In the case where multi-beammodules are included in the system, it is preferred when there is no gapbetween the individual modules. Thus, the amount of cross talk to thenext lens field is similar for all lens fields. In the embodiment ofFIG. 5, a lens module 501 is shown with five round openings 502 for fiveelectron beams traveling along the five optical axes, respectively.

Further embodiments will now be described with respect to FIGS. 6 to 8.FIGS. 6 a and 6 b show a linear lens array comprising five lens modules.Each lens module has a conductive material circuit 23 in the form of acylinder with two flattened sides. In the middle of the magneticmaterial circuit, opening 22 is provided. An electron beam travelingalong the optical axis 24 through opening 22 is focused by a magneticlens field. The magnetic lens field is induced by currents 106 a and 106b.

As described above, each individual lens module has two planes ofsymmetry with respect to optical axis 24. In FIG. 6 a, in order tomaintain the symmetry of the modules, which might partly be distorted bycross talk, there is a gap 62 in-between each module. Preferably, thegap is between 0.1 mm and 3 mm. Since the magnetic resistance of themagnetic-conductive circuit is several magnitudes lower than themagnetic resistance in the gap, cross talk between neighboring lensmodules can be neglected. In an alternative embodiment shown in FIG. 6b, gap 62 is filled with a non-magnetic material. Thus, plate 64 acts asa magnetic isolator between neighboring lens modules. Consequently, thesymmetry of each lens module is undisturbed.

A further embodiment of the present invention is shown in FIG. 7 a. Thelens modules 100 are arranged side by side. The symmetry of each lensmodule is maintained according to the following arguments. In the caseof an infinitely long linear lens array, each lens module 100 would havean identical neighborhood. Thus, each module would have a two-foldsymmetrical lens field and no dipole moment. In practice the linear lensarray is not infinitely long. Therefore, especially lens modules at theends of the linear array could have a distorted symmetry. According toFIG. 7 a, additional lens modules 700 are provided. These additionalmodules are not used to focus an electron beam and can be considered a“dummy” module. However, their symmetry is not relevant for the beamforming property of the other lens modules of the linear lens array. Asshown in FIG. 7 a, five modules, each with one opening for a chargedparticle beam, are used for focusing. Additionally, two “dummy” modules700 are provided. Concerning the number of modules and additional lensmodules 700, such an embodiment can be considered preferred. However,the present invention should not be limited, thereto. Instead, more thanfive modules for charged particle beams can be applied and more than one“dummy” modules can be located at each side.

A further embodiment is shown in FIG. 7 b. The embodiment comprisesshielding plates 702 at both ends of the linear lens array. Theseshielding plates can be combined with the “dummy” modules 700 shown inFIG. 7 a or can be used independently of the “dummy” modules 700. Theshielding plates have two different effects. On the one hand, theinfluence of the loop of the excitation currents at the end of thelinear lens array is shielded. On the other hand, a magneticneighborhood (periphery) can be provided as if the linear lens arraywere infinitely long. These two effects can be understood as follows.

The straight conductor for the excitation current cannot be continuedarbitrarily for an infinite length. Thus, the conductors have to form aloop at both ends of the linear lens array. In general, the conductorsfor a linear lens array can be described as a rectangle, whereby theedges of the rectangle are rounded. The conductors comprise severalwindings to form an excitation coil. The radius of curvature at therounded edges can be increased until the conductors for the excitationcurrent form a semi-circle at both ends. Thereby, a stadium-like form isrealized. This semi-circle disturbs the symmetry of the system. Ifshielding plate 702 is introduced, the lens modules 100 are notinfluenced by the semi-circle conductor loop at both ends of the linearlens array.

The other effect of the shielding plates is the following. In the casewhere the symmetry is based on the fact that every lens module has asimilar magnetic surrounding, the end of the linear array distorts thesymmetry. Thus, the shielding plate can be used to provide a magneticsurrounding for lens modules located at the end of the linear array asif the linear array would be continued. Consequently, using shieldingplates 702, the symmetry distorted by the finite length of the row ofopenings can at least partly be recovered.

The aspects presented above, namely to provide dummy modules 700 or toprovide shielding plates 702, can be used independently for all kinds oflens modules. Thus, the usage of dummy modules 700 or to provideshielding plates 702 within the present invention is not limited to thetype of lens modules shown in FIGS. 7 a and 7 b. It is apparent thatthese aspects can be combined with all kinds of linear lens arrays.

In the embodiment shown in FIG. 8, the lens modules 100 are manufacturedto shape the magnetic flux provided to opening 22. Therefore, additionalholes 82 are provided, whereby the magnetic field provided to opening 22is modified. Making use of the additional holes 82, it is possible tocustomize the field focusing the electron beam. In FIG. 8, two openings82 above and below opening 22 are provided. Thus, the n-fold symmetrywith respect to optical axis 24 with n>1 (no dipole moment) ismaintained.

In context of this invention, the aspect of providing additionalopenings 82 can be combined with aspects of other embodiments presentedin the application.

A cross sectional view similar to a view along symmetry plane 104 inFIG. 1 a of a further embodiment is shown in FIG. 9. According to thisembodiment, an electrostatic lens is provided within opening 22. Theelectrostatic lens comprises two electrodes 92 and 94 arrangedsymmetrically with respect to optical axis 24. The two electrodes areused as an electrostatic immersion lens, whereby the imaging propertiescan be improved. The electrostatic lens within opening 22 according tothe embodiment of FIG. 9 can be combined with other aspects of thevariety of embodiments presented.

Generally, the embodiments described above refer to the followingaspect. At least two lens modules are provided. These lens modules arearranged in the form of a linear lens array, a lens row, and share asingle excitation coil. Thereby, a continuous rotational symmetry isdeliberately abandoned. Instead, an n-fold rotational symmetry withrespect to the optical axis is obtained (n<∞). Since the symmetryobtained has no dipole moment, n is greater than one (1<n<∞). Thesymmetry can also be described by two planes of symmetry, bothintersecting optical axis 24.

The above described symmetry can either be achieved by avoiding crosstalk of neighboring lens modules or by providing a quasi infinite lineararray lens. Quasi infinite should be understood as either providing alinear array of a sufficient length such that lens modules not locatedat one of the two ends of the linear array are influenced as if thelinear array were infinitely long, or should be understood as providingcorrection means as shielding plates or “dummy” modules to simulate aninfinitely long linear array.

FIG. 10 a shows a further embodiment of a lens module 100. Themagnetic-conductive circuit 23 has two flattened sides. Due to theflattened sides, two lens modules can be located closer together. Forthis reason, electron beams traveling through the opening 22 have asmaller distance. The lens module 100 has two planes of symmetry 102 and104. Thus, a lens field has no dipole moment with respect to opening 22.

Compared to the circular opening 22 in FIG. 10 a, the embodiment shownin FIG. 10 b has a rectangular opening 22 a. This rectangular opening 22a does not generally change the symmetry conditions. However, the lensfield influencing an electron beam traveling along the optical axis(center of opening 22 a) is modified as compared to the circular openingin FIG. 10 a. The choice of the shape of the opening can be used toshape the magnetic flux influencing the electron beam. Thus, themagnetic lens field can be customized.

Both embodiments shown in FIG. 10 have two planes of symmetryintersecting the optical axis. The form of the magnetic-conductivecircuit result in a 2-fold symmetry with respect to the optical axis. Inthe case where the magnetic-conductive circuit 23 is, according toanother embodiment (not shown), a square, a 4-fold symmetry is obtained.

Further embodiments according to the present invention are shown inFIGS. 11 a and 11 b. The magnetic-conductive circuit 23 shown in FIG. 11a has a hexagonal shape. Opening 22 b also has a hexagonal shape, whichhas the same orientation as the magnetic-conductive circuit 23. Due tothe hexagonal shape, three planes of symmetry 102 a, 102 b and 102 cexist. This symmetry can also be described as a 6-fold symmetry withrespect to the center of opening 22 b. FIG. 11 b shows amagnetic-conductive circuit in the shape of an octagon. Octagon-shapedopening 22 c is rotated by 22.5° with respect to octagon 23. The 8-foldsymmetry of the embodiment of FIG. 11 b is unaffected by this rotationof opening 22 c. The rotation is another example for a flux shapingtechnique. The 8-fold symmetry can also be described by the four planesof symmetry 102 a to 102 d.

All embodiments shown in FIGS. 10 a to 11 b have at least a 2-foldsymmetry with respect to the optical axis. The magnetic-conductivecircuits and openings described have up to 4 planes of symmetry, eachintersecting the center of the respective opening. According to afurther embodiment, the increasing number of symmetry planes related tothe form of the magnetic-conductive circuits can be decreased by theform of the excitation. FIG. 12 shows an embodiment with a circularmagnetic-conductive circuit 23. Coil 21 used for several lens moduleshas straight conductors, which are in contact with themagnetic-conductive circuit. Thus, the system comprising the lens moduleand the excitation coil has a 2-fold symmetry. The semi-circle of coil21 at the two ends is not taken into account, since it can be shieldedas described with respect to FIGS. 7 a and 7 b.

Generally, the embodiments described above have magnetic-conductivecircuits 23 with at least two planes of symmetry. Combining thesemagnetic-conductive circuits with straight conductors, the order ofsymmetry is reduced to two planes of symmetry. The shape of themagnetic-conductive circuits according to embodiments described abovecan be combined with other aspects of the variety of embodimentspresented within this application.

FIG. 13 shows a linear lens array 130 comprising five lens modules 100.Each lens module comprises a magnetic-conductive circuit 23 forming polepieces. Within the oval opening 22 in each lens module, a magnetic lensfield for focusing an electron beam traveling on the optical axis(=center of opening 22) can be provided.

At both ends of the linear lens array 130, there is a shielding plate702. Shielding plate 702 shields the excitation current of thesemi-circles of coil 21 at both ends. Further, the gap between each lensmodule is filled with a non-magnetic material forming a magneticisolator 64. Thus, the lens modules do not influence each other and the2-fold symmetry of each module is not distorted.

To increase the number of electron beams focused by the lens unit,several linear lens arrays 130 can be arranged next to each other. Arespective system is shown in FIG. 14. Thereby, the imaging propertieswith respect to the symmetry of the lens fields and the manufacturingconditions can be improved. According to an embodiment shown in FIG. 14,the reduced charge particle beam distance of 10 mm to 90 mm can berealized in one direction. In the other direction a charged particlebeam distance of 100 mm to 200 mm can be realized.

Within FIG. 14, the currents in coils 21 are indicated by the fourarrows 142 a to 142 d. As can be seen from the figure, the two currents142 b and 142 c in the middle of the two linear lens arrays are parallelbut directed in the opposite direction. Bringing the two linear lensarrays close together it can be deduced that the total excitationcurrent in the middle of the system is zero, since currents 142 b and142 c are canceled out. Thus, it is possible to leave out the tworespective regions of coils 21 being directly adjacent. Thus, for thetwo linear rows of lens modules, a single excitation coil can be used,as shown in FIG. 15.

Thereby, it is preferred to have no magnetic isolating material inbetween the two rows. As can be seen from FIG. 15, each opening 22 hasonly one conductor for the excitation current extending within thedirect vicinity of the opening. Thus, the symmetry with respect to eachopening is disturbed. However, the embodiment shown in FIG. 15 can stillbe manufactured more easily, since the lens modules are formed ofsmaller pieces of μ-metal or permalloy as compared to a lens unit havingseveral openings in a single piece. Since these smaller material blocksare easier to provide and easier to process, the embodiment shown inFIG. 15 is still advantageous with respect to prior art systems.

The embodiment shown in FIG. 15 was deduced by leaving out the regionsof the coils wherein the currents canceled each other out. The sameargument can be applied to combine more than two rows of modules and usea common excitation coil.

Generally, the aspect of forming more than one row of openings bypositioning several linear lens arrays next to each other is not limitedto the kind of lens module 100 described with respect to FIGS. 13, 14and 15. The shape of the lens module, the shape of opening 22, thepositioning of the lens modules with or without a gap, as well as theexistence of a non-magnetic material within a possible gap or theexistence of shielding plates 702 can be varied. The aspects ofcombining several linear lens arrays can be combined with all kinds oflinear lens arrays described within this application.

The previous embodiments referred to a magnetic lens comprising at leasttwo lens modules. The lens modules share a common excitation coil.According to the shown embodiments of linear lens arrays, the symmetryof the lens field is at least a 2-fold symmetry with respect to eachindividual optical axis. This could be achieved by either providing lensmodules without any cross talk to neighboring lens modules, lens moduleseach having similar cross talk compared to the other lens modules, orhaving a quasi-infinite linear lens array.

The aspect of having at least a 2-fold symmetry with respect to eachoptical axes can—in the case of a quasi-infinite linear lens array—alsobe realized independently of the number of modules sharing a commonexcitation. Thus, it is possible to have a linear lens array providingmagnetic fields to each opening for an electron beam that has a symmetrycomparable to a infinitely long linear lens array.

An embodiment according to this aspect is shown in FIG. 16. Therein,pole piece unit 501 has five openings. The center of each openingprovides an optical axis for an electron beam. Within the pole pieceunit 501, straight conductors are provided as an excitation current. Theconductors for the excitation current form a loop within end pieces 162.On the one hand, end pieces 162 shield the pole piece unit from themagnetic field formed by the loops of the excitation current. On theother hand, a magnetic flux can be shaped as if the linear lens array iscontinued infinitely long, that is to say, a magnetic flux of anquasi-infinitely long linear lens array is shaped.

According to a preferred aspect of the present invention, the length ofthe pole piece unit can be increased. Thereby, the number of openingscan be increased. If a sufficient number of openings is provided,openings adjacent to end piece 162 do not need to be used for focusingan electron beam. These openings can be used as dummy openings. Thus,all openings used for focusing an electron beam are approximatelyinfluenced by the same periphery.

Another embodiment according to this aspect is shown in FIG. 17. Lenssystem 170 comprises a plurality of openings 22 for focusing electronbeams traveling on the optical axes. The openings 22 are provided in aelongated magnetic-conductive circuit 172. The shape of the lens systemis oval. The curvature of the oval is chosen such that within eachopening a magnetic lens field with at least a 2-fold symmetry isprovided. To further shape the magnetic flux, two dummy openings 82 areprovided adjacent to each opening 22. Each lens opening 22 and therespective dummy openings are positioned along a line.

1. A lens system for a plurality of charged particle beams, comprising:at least two lens modules, each comprising a first pole piece, a secondpole piece and at least one opening for a charged particle beam; and atleast one excitation coil providing a magnetic flux to the at least twolens modules, wherein each lens module constitutes a component.
 2. Thelens system according to claim 1, wherein one charged particle beamtravels through each of the openings, thereby being focused in a lensfield area.
 3. The lens system according to claim 1, wherein the centerof each opening provides an optical axis and wherein a lens fieldcorresponding to each opening has at least two planes of symmetry withrespect to its optical axis.
 4. The lens system according to claim 1,wherein the openings of all lens modules sharing one excitation coilform a row of openings.
 5. The lens system according to claim 1, whereinat least four openings are provided within one row, thereby increasingsymmetry for each opening with respect to its optical axis.
 6. The lenssystem according to claim 1, wherein the at least one excitation coilhas a non-circular shape.
 7. The lens system according to claim 1,wherein the at least one excitation coil has substantially the shape ofa rectangle with rounded edges.
 8. The lens system according to claim 1,further comprising at least two lens rows, each comprising an excitationcoil; and at least two lens modules arranged next to each other to forma two-dimensional arrangement of openings.
 9. The lens system accordingto claim 1, wherein the at least two lens modules are arranged to form atwo-dimensional arrangement of at least four openings, and therebysharing one excitation coil.
 10. The lens system according to claim 1,wherein the openings for the charged particle beams have at least in onedirection a distance with respect to each other of about 10 mm to about90 mm.
 11. The lens system according to claim 9, wherein each row oflens modules is terminated at its ends by a shielding plate.
 12. Thelens system according to claim 1, wherein each lens module is positionedin relation to an adjacent module by providing a gap of about 0.1 mm to3 mm.
 13. The lens system according to claim 12, wherein the gapcontains a non-magnetic material.
 14. The lens system according to claim1, wherein each lens module comprises magnetic flux shaping openings.15. The lens system according to claim 1, wherein for each magneticsub-lens, an electrostatic immersion lens is provided.
 16. The lenssystem according to claim 15, wherein each electrostatic immersion lenscomprises at least two electrodes.
 17. A method for manufacturing a lenssystem, comprising: manufacturing a plurality of lens modules, eachcomprising a first pole piece, a second pole piece and at least oneopening for a charged particle beam; and providing a common excitationcoil for at least two lens modules.
 18. The method according to claim17, wherein each module is manufactured by first providing a cylindricalintermediate product and then flattening at least two sides of thecylindrical intermediate product.
 19. (canceled)
 20. (canceled)
 21. Alens system for a plurality of charged particle beams, comprising: anexcitation coil providing a magnetic flux to a pole piece unit having afirst pole piece, a second pole piece and at least two openings forcharged particle beams; wherein the two openings are arranged in onerow, thereby forming a lens row; and wherein the pole piece unit has anelongated shape.
 22. The lens system according to claim 21, wherein theexcitation coil has a non-circular shape.
 23. The lens system accordingto claim 21, wherein the excitation coil has a rectangular shape withrounded edges.
 24. The lens system according to claim 23, wherein theedges are rounded such that the sides of the rectangular shape form asemi-circle.
 25. The lens system according to claim 21, wherein any ofat least four openings are provided within one row, thereby increasingsymmetry for each opening with respect to its optical axis.
 26. The lenssystem according to claim 21, wherein one charged particle beam travelsthrough each of the openings, thereby being focused in the lens fieldarea.
 27. The lens system according to claim 21, wherein the center ofeach opening provides an optical axis and whereby a lens fieldcorresponding to each opening has substantially at least two planes ofsymmetry with respect to its optical axis.
 28. The lens system accordingto claim 21, wherein at least two lens rows, each comprising anexcitation coil, are arranged next to each other to form atwo-dimensional arrangement of openings.
 29. The lens system accordingto claim 21, wherein the openings for the charged particle beams have atleast in one direction a distance with respect to each other of about 10mm to 90 mm.
 30. The lens system according to claim 21, wherein eachlens row is terminated at its ends by a shielding plate.
 31. The lenssystem according to claim 21, wherein the pole piece unit comprisesmagnetic flux shaping openings.
 32. The lens system according to claim21, wherein for each magnetic sub-lens, an electrostatic immersion lensis provided.
 33. The lens system according to claim 32, wherein eachelectrostatic immersion lens comprises at least two electrodes.
 34. Amultiple charged particle beam device, comprising: a charged particlebeam source; a detector for detecting secondary particles; beam shapingmeans; a housing for the charged particle beam column, wherein thehousing can be evacuated; at least one lens system comprising: at leasttwo lens modules, each comprising a first pole piece, a second polepiece and at least one opening for a charged particle beam; and at leastone excitation coil providing a magnetic flux to the at least two lensmodules, wherein each lens module constitutes a component.